Polyvinylidene fluoride (PVDF) is widely used in the preparation of microfiltration and ultrafiltration membranes due to its good thermal stability, chemical resistance, excellent processability and convenience in controlling the porosity and the morphology. However, applications of these membranes for water purification have been limited due to the hydrophobic nature of PVDF, where fouling occurs on membrane's surfaces and pores.
A variety of constituents in water can lead to membrane fouling, including dissolved inorganic or organic compounds, colloids, bacteria and suspended solids. Biofouling is largely attributable to accumulated extracellular materials, rather than individual bacterial cells or microbial flocs. These extracellular materials, including soluble microbial products and extracellular polymeric substances, consist mainly of polysaccharides, proteins and natural organic matter.
In recent years, hydrophilic microfiltration and ultrafiltration membranes prepared from chemically or physically modified PVDF have been widely studied and reported. Several approaches have been developed to endow the membranes with hydrophylic properties, such as surface coating and surface grafting.
For instance, WO 2012/175416 (SOLVAY SPECIALTY POLYMERS ITALY S.P.A.) Dec. 27, 2012 discloses a process for the manufacture of porous membranes, said process comprising providing a composition comprising at least one fluoropolymer, said fluoropolymer comprising recurring units derived from at least one (meth)acrylic monomer, and at least one poly(alkylene oxide). The poly(alkylene oxide) has a number average molecular weight typically comprised between 100000 and 5000000.
Also, US 2007/0219322 (MASSACHUSETTS INSTITUTE OF TECHNOLOGY) Sep. 20, 2007 discloses a process for the manufacture of graft copolymers via a controlled free-radical process by grafting hydrophilic chains onto polymers, particularly hydrophobic polymers such as polyvinyl chloride), PVDF and chlorinated polypropylene. The hydrophilic chain can comprise a poly(ethylene oxide). Membranes for water filtration can be prepared from the graft copolymers thereby provided.
Nevertheless, coating or surface grafting directly on a membrane has some shortcomings. For example, a coated surface layer that is physically absorbed on the membrane's surface can be easily washed off and surface grafting of membranes is likely to be accompanied by changes in membrane pore size and distribution, which results in reduced permeability.